1. Field of the Invention
The field of the present invention relates generally to implanted devices for tissue stimulation, monitoring, and other therapeutic or diagnostic functions, and specifically to implantable devices for the stimulation of cardiac tissue, for example pacemakers or implantable cardioverter-defibrillators (ICDs). More specifically, it pertains to such devices utilizing wireless energy transfer, for example using ultrasound energy.
2. Description of the Background Art
Pacemakers provide electrical stimulus to heart tissue to cause the heart to contract and hence pump blood. Conventionally, pacemakers include a pulse generator, typically implantable in a patient's pectoral region, with one or more leads (wires) extending from the pulse generator into a heart chamber. The lead terminates at an electrode, which is implanted in the heart.
While pacemakers using leads are widely used, they have several drawbacks. For example, the gradual intertwining of leads with heart tissue over time secures the lead in place but also hinders lead removal or repositioning. Another drawback to using leads is the limit placed on the number of heart sites that may be stimulated. While pacing at multiple sites may be beneficial for treating different heart conditions such as congestive heart failure, arrhythmia and atrial fibrillation, using multiple leads may block a clinically significant fraction of the cross section of the veins and cavities through which the leads are routed.
Pacing systems using wireless electrodes have been suggested as a way of overcoming the limitations of conventional systems with leads, with wireless receiver-stimulator electrodes implanted into the heart wall and in wireless communication with transmitter(s) for energy delivery or for communication of control or feedback signals. The inventors of this patent application have proposed systems using implantable wireless electrodes that receive acoustic energy and convert it into electrical energy for electrically stimulating the heart. Such methods and systems have been disclosed in co-pending U.S. Patent Application Nos. (Publication No.) 20060136004, 20060136005, 20070027508, 20070055184, 20070078490 and 20070060961 and Ser. No. 11/752,775, which are herein incorporated by reference in their entirety. As another example, U.S. Patent Application No. (Publication No.) 2006/0085039 discloses a system using implantable wireless electrodes that receive energy via inductive coupling of a coil in the electrode to a radio frequency antenna attached to a central pacing controller.
When implanting a wireless receiver-stimulator, the choice of the implantation location is important for at least two reasons. First, it is desirable that the tissue in electrical contact with the stimulation electrodes of the receiver-stimulator be sufficiently excitable to allow efficient pacing stimulation by the receiver-stimulator. Secondly, it is desirable that the wireless receiver-stimulator be positioned relative to the wireless transmitter to allow efficient wireless communication between the two, particularly with respect to energy transmission and reception.
While the determination of the location in conventional systems with leads involves fairly straightforward techniques, such techniques do not translate directly for wireless pacing systems. In a conventional pacing system, determination of an excitable tissue location is customarily practiced by monitoring electrogram (EGM) signals at the implantation site and additionally by stimulating or pacing through the electrodes, before permanently implanting them in the patient. The user simply connects the proximal end of the pacing lead into a pacemaker programmer or other electrophysiology instrumentation that allows the user to monitor EGM signals from the electrodes on the lead and to stimulate through the electrodes on the lead to confirm that the implant location is appropriate.
In contrast, in a wireless system one obstacle is the lack of a direct connection to one or more of the electrodes for the monitoring of EGM signals. Additionally, stimulating through the wireless electrodes involves transmission of energy from a transmitter to a receiver-stimulator through a wireless process, whether for charging the receiver-stimulator or for transduction from wirelessly delivered energy to stimulation energy. This lumps two effects together: the efficiency of the wireless transfer of energy (by whatever means the system employs, such as acoustic energy, radio frequency (RF), or other means) and the properties and excitability of the tissue that the pacing electrodes are placed over. This could result in user confusion and potentially inaccurate determination of pacing thresholds and energy conversion efficiencies.
For example, in a conventional pacing system with leads, a high pacing threshold implies a poor location for placing the pacing electrodes. This may indicate, for example, that the electrodes are not in close proximity to the tissue or are placed over non-excitable tissue. A straightforward resolution of this problem is moving the electrode until an appropriate location is found. In contrast, in a wireless system a high energy level that is required to pace could be the result of inefficient or poor wireless transfer of energy from the transmitter to the receiver-stimulator, or, similar to the conventional pacing system, the result of a poor location of the receiver-stimulator not in close proximity to the tissue or over non-excitable tissue.
An extension of the above lead-based techniques to wireless stimulation systems comprises establishing electrical contact between one or more of the electrodes of an implantable wireless receiver-stimulator, a delivery system (such as a catheter or the like), and the tissue. Alternatively, surrogate electrodes on the delivery system, i.e., not the electrodes of the wireless receiver-stimulator, may be used for assessing whether the tissue is excitable. However, it requires the use of one or more of the electrodes of the implantable wireless receiver-stimulator to fully assess the efficient and effective transfer of energy to the receiver-stimulator from the transmitter. The desirable approach is to use one or more of the electrodes of the implantable wireless receiver-stimulator to sense local tissue EGMs in order to (1) determine a suitable implant location, as well as (2) determine efficiency of energy conversion by the wireless implant. For example, such a technique is partially suggested in the above referenced U.S. Patent Application (Publication No.) 2006/0085039. Another approach to determine the appropriate location for implantation of the electrodes is to observe the hemodynamic parameters of the heart upon stimulating a location. Such an approach is described in the Applicants' co-pending U.S. Patent Application (Publication No.) 2007/0060961. While this desirable approach may be constructed, it does give rise to a number of challenges.
First, once the wireless electrode is implanted, disconnected from the delivery system, and the delivery system is removed, any conductive material at the severed connection on the wireless implant, remaining exposed after disconnecting the delivery system from the electrode(s), presents a potential alternate electrical path between the implant electrodes and the exposed remains, allowing some or all of the stimulation current to bypass the desired stimulation path and thereby reduce or entirely undermine stimulation effectiveness.
Second, it is also desirable to be able to assess conversion efficiency in-situ, perhaps over a variety of energy transmission conditions. It would be desirable to perform this assessment while directly connected to one or more of the electrodes without requiring that the wireless implant deliver electrical output (stimulation energy) at sufficient strength to capture tissue. By monitoring the electrical energy output, the efficiency of transmission can be assessed and the likelihood of pacing capture can be correlated with the efficiency.
Therefore, it is desirable to have a wireless pacing system that allows the user to determine a suitable implant location and assess the efficiency of energy conversion prior to permanent implantation by using the pacing electrodes of the receiver-stimulator, and further eliminate exposed residual conductive material after removal of the delivery system.